LONDON — British authorities began combing through their intelligence files and evidence from the attack site Thursday to determine whether the apparently terror-related killing of a soldier on a London street could have been prevented.

As political and community leaders vowed not to be cowed by the vicious assault, Scotland Yard announced the arrest of two new suspects in addition to the alleged attackers. The new suspects, a man and a woman, both 29, were arrested on suspicion of conspiring to murder. Investigators gave no further details.

Officers also searched several homes thought connected to the two men who allegedly hacked the off-duty soldier to death in front of stunned bystanders and spouted Islamic political statements before being shot and wounded by police.

British news outlets reported that both suspected assailants had previously come to the notice of security agencies but were not judged to be planning an attack. The BBC named one of them as 28-year-old Michael Adebolajo, a British citizen of Nigerian descent who the broadcaster said came from a devout Christian family but converted to Islam.

Adebolajo was identified as the man seen in videos brandishing knives in his bloodied hands after the attack and declaring the killing to be payback for the deaths of Muslims in countries where Britain has deployed troops. His alleged accomplice, who police said is 22 years old, has not been named.

The dead soldier was Lee Rigby, a drummer and machine-gunner with the Royal Regiment of Fusiliers who had seen duty in Afghanistan, said Britain’s Defense Ministry. Rigby’s battalion is based in barracks next to the site of the attack, in the Woolwich district of southeast London.

“We have lost a brave soldier,” said Prime Minister David Cameron, adding: “The people who did this were trying to divide us. They should know something like this will only bring us together and make us stronger.”

Rigby, 25, is the first person to have died on British soil in an apparent attack by Muslim extremists since the 2005 suicide bombings on London’s transit system, in which 52 people died. Authorities have since managed to thwart a number of plots of mass killing by Islamic terrorists, many of them “homegrown” radicals, as were the transit bombers.

Investigators are eager to question the two alleged assailants to determine whether they acted in league with a terrorist network such as al-Qaeda. Parliament’s intelligence committee announced that it would conduct an inquiry into what security agencies knew about the men prior to the assault.

British news reports said that one of the suspects was stopped by authorities last year from traveling to Somalia to join the radical al-Shabab group.

Police assigned an extra 1,200 officers to patrol London on Thursday. Despite public shock over the hacking death, the British capital — which has been the target of terrorist attacks by Irish republicans and Islamic militants for decades — seemed much as usual.

“Londoners can go about their business in the normal way,” Mayor Boris Johnson said after attending an emergency-response committee meeting at the prime minister’s office Thursday morning.

The two primary suspects remained hospitalized in stable condition and were under armed guard.

The government’s official assessment of the threat posed by international terrorism was unchanged at “substantial,” meaning that an attack remains a strong possibility.

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